Osteoporosis is a major public health problem. In this application, we propose to extend our study of the effects of weight-bearing physical activity and a high calcium diet on increases in bone quality in pubertal girls. Since there are few known safe, effective methods for restoring lost bone to the osteoporotic skeleton, prevention of osteoporosis is crucial. If skeletal development can be maximized during growth, young people will begin adulthood with optimal bone quality and be less likely to develop osteoporosis in later years. Yet bone health of youth has not been adequately addressed. The aims of this experimental study are 1) to test the effect of a weight-bearing exercise program on increases in bone quality in adolescent females; and 2) to compare the effect of a weight-bearing exercise program with the effect of a weight-bearing exercise program and a high- calcium diet on increases in bone quality in adolescent females. The primary hypotheses to be tested are: 1) Adolescent females who have completed a weight-bearing exercise program will have higher spine BMC and calcaneal SOS than adolescent females who have maintained their usual activity level, and 2) adolescent females who have completed a weight-bearing exercise program and a high calcium diet intervention of 1500 mg/day will have higher spine BMC and calcaneal SOS than adolescent females who have completed the exercise program but have maintained their usual calcium intake. We will follow 107 females ages 10-13 years for an additional four and one-half years. They have been randomly assigned to 1 of 3 groups: 1) those in a weight-bearing exercise program which meets 3 times/week and who consume their usual diets (current n=35); 2) those in a weight-bearing exercise program which meets 3 times/week and are given high calcium foods to supply 1500 mg/day (current n=35); or 3) a control group who consumes their usual diet and maintains their customary activity level (current n=37). Measurements of spine, hip, radius and total body BMC, calcaneal SOS, height, weight, Tanner stage, and medical and social history will be made on all participants semiannually and at the end of study. Usual dietary intake and physical activity will be assessed every three months and at the end of study. Interim analysis of data from our study suggests that the combination of a high calcium diet and regular weight- bearing physical activity significantly increases bone mass gain compared to physical activity alone or to a control group. If these findings hold, they would provide strong support for the essential role of calcium in addition to exercise in maximizing bone mass in growing females.